It's tornado season in Colorado. After a tornado has touched down and caused damage, it's up to the National Weather Service to determine its intensity. Meteorologists do this using a scale called the Enhanced Fujita, or EF, scale.
The tornadoes that hit portions of the Denver metro region on Sunday were significant. Many homes were damaged or destroyed. After the storm, a storm survey is conducted. The degree of damage to homes and trees determines the intensity of the storm.
Let's look at a tornado moving through different areas. First, it downs some oak trees. Then it breaks some pine trees. Finally, it removes a chimney from a brick home. Meteorologists know the wind speeds required to damage different types of objects, from minor damage to destruction.
In this example, the oak trees would require winds between 100 and 130 miles per hour. The pine trees would need 90 to 120 mile per hour wind. And the chimney would need 80 to 115 mile per hour winds.
We can only estimate wind speed up to the destruction of an object. For a pine tree, 130 miles per hour winds would snap or uproot the tree and debark it - which is the maximum damage a pine tree can have. So even if wind speeds were actually 170 mph hitting that tree... the tornado would earn a 130-mile-per-hour rating for that damage.
The EF scale assigns an intensity rating based on the estimated wind speeds generated by the damage the tornado caused. The scale goes from 0 to 5. A weak EF zero has winds of 65 to 85 miles per hour, while a catastrophic EF five packs winds above 200 miles per hour! The rating the tornado gets is for whatever damage indicator requires the highest winds. In our case, it was the oak trees. Their destruction earns this tornado an EF2 rating with winds of 120 miles per hour. The chimney damage would've only been rated EF-1.
This is how meteorologists can be so precise in estimating tornado wind speeds. We look at hundreds of objects, the way they are damaged, and compare that against wind speeds required. NWS storm surveyors use multiple data points to determine wind speeds. For example, it might take wind speeds of 100-130 mph to down an oak tree. But, let's say that oak tree was next to a house, and the chimney only sustained partial damage - requiring 90-110 mph winds. The storm survey would take those two pieces of damage together and conclude wind speeds were likely around 110 mph - a strong EF-1.
EF-4 and EF-5 tornadoes are the most catastrophic, with wind speeds above 166 miles per hour. Fortunately, they're rare. They only make up half a percent of all tornadoes in the U.S. Despite their rarity, they're responsible for half of U.S. tornado deaths, according to data from the Storm Prediction Center. These tornadoes completely destroy well-built brick homes that are anchored to their foundations. Again, it's important to note that a tornado must hit and destroy a well-built building to earn an EF-4 or EF-5 rating. Even if a tornado has wind speeds that would classify it at that intensity, if the tornado is over an open field, it will not earn that EF-4 or EF-5 rating.
Most tornadoes in southern Colorado are weak, but strong tornadoes have occurred. Nine tornadoes of EF-2 or stronger intensity have hit El Paso County alone since 1955. The most notable was an EF-3 that tracked from Manitou Springs along U.S. 24 into the western part of downtown Colorado Springs on June 24th, 1979. And an EF-2 tornado hit the south side of Pueblo in August of 1984.
The Palmer Divide and eastern plains both have vastly higher counts of both weak and strong tornadoes. The point here is we do get strong tornadoes in southern Colorado, even in our cities. They're rare, but they do occur. The peak in Colorado tornado activity is in early June climatologically.
Because we rate tornadoes on damage rather than measured wind speeds, some strong tornadoes don't get reported as such. A tornado with 200 mph winds over an open field won't hit anything that requires 200 mph wind to be completely obliterated. It might end up earning an EF-1 rating for downing some trees, or an EF-U (EF unknown). Therefore, if you see a tornado when driving or outside, don't assume it's weak and not a big deal. Avoid it. Sunday's severe weather is a reminder that it's smart to have a severe weather plan in place and know where to shelter if a tornado warning is issued.
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